This invention is an improvement in the type of dishwasher shown in U.S. Pat. No. 3,323,529 issued to Geiger et. al. In that patent, there is disclosed a system for pumping filtered fluid through a wash arm by means of a recirculating pump while the sump of the dishwasher is being drained, so that descending wash solution or rinse water directs food particles and other debris toward the drain until the sump is almost completely drained. To accomplish this result, the flushing action relied both on falling water from the upwardly directed spray issuing from the wash arm and on the normal turbulence of draining water. While the flushing system of the Geiger patent was a considerable improvement over then-existing systems, the design and shape of the filtering screen, plus the tenacity of certain types of food particles to cling to the underside of the screen disclosed in that patent made complete flushing difficult, and required occasional screen removal from the machine for cleaning, depending on the soiled condition of dishes normally placed in the washer by the operator.
Providing a recirculating pump with a filtering system around the pump intake, and providing additional means for utilizing a portion of the filtered fluid to flush debris from the filtering system, is known from U.S. Pat. No. 2,552,493 issued to Newton, U.S. Pat. No. 3,090,391 and 3,491,780 granted to Kaldenberg, and U.S. Pat. No. 3,575,185 granted to Barbulesco. In a typical such dishwashing machine the fluid is filtered and recirculated under pressure to spray the dishes for removing the food soil. The filtering is by a fine mesh screen, and it is especially desirable that the mesh of the screen be very fine, so that recirculated liquid which contacts the dishes will contain less and less soil as washing and subsequent rinsing progress. This presents a problem, however, in that the finer the screen, the quicker the tendency toward clogging and starvation of the recirculating pump. This is due to the reduced ability of the screen to pass sufficient water as it becomes increasingly clogged and impervious, depending of course on the amount and type of food soil on the dishes. The approach of the aforementioned prior art patents was to provide a high velocity stream of water internally of the screen to break loose any food soil which might be clinging to the outside of the screen. A portion of the filtered water was therefore directed upstream, backwardly through the fine screen to dislodge debris and food particles clinging thereto. Of course, the stream had to have sufficient pressure to overcome the negative pressure within the screen caused by suction at the pump inlet, and it was hoped the loosened soil would descend to the bottom of the sump and into the drain section for draining at the completion of that particular portion of the machine cycle.
However, the turbulence of water in the sump and the nature of the food particles in suspension in the water do not always permit such particles to lie in a quiescent state at the sump bottom. On the contrary, smaller particles are likely to stay in suspension and be repeatedly drawn into contact with the outer surface of the fine screen and again backflushed away from the screen, because of the nature of the structure providing the backflushing. This constant working, both as a result of the repetitive jet pressure of the backflow and of the mechanical action caused by the repeated intermittent contact of food particles with the screen, tends to disintegrate the particles, reducing their sizes so that many may become small enough to pass through the screen openings and be recirculated. The undesirable result of such a breakdown in the size of the particulate matter is the increased possibility of continuous redeposition thereof onto the dishes throughout the remainder of the dishwashing and rinsing cycles. This would considerably increase the likelihood of leaving minute food particles on the dishes at completion of the total cycle, thus requiring additional rinsing and the attendant excessive use of water to overcome this problem.
A further purpose of the so-called self-cleaning filtering systems of the aforementioned prior art, i.e., in addition to attempting to keep the screen sufficiently clean to enable passage of water therethrough, is to flush food soil off the screen and down the drain at the completion of each washing or rinsing period. This reduces the frequency of removing the fine screen from the dishwasher for hand cleaning.
In addition, the Kaldenberg and Barbelusco patents show either a submerged rotating jet mechanism or a submerged rotating screen to accomplish the desired backflushing action. Because they are submerged, they can be expected to require somewhat more driving force than would be necessary if the flushing elements were above the water level. For the most part, such prior art machines also require auxiliary devices to provide the cleaning action on the screen, rather than making use of equipment which is already present in most dishwashers of this type.